Thanks for the thorough, thoughtful reply, Megatron...I'll jump in on your comments, it would be great if others would join, too!
As for history, the RWD Chrysler A727 Torqueflite transmission has seen a lot of severe service and so has its overdrive A518 and 42RE to 48RE truck derivatives! By the time our trucks were built, Chrysler boasted that the 48RE was the best unit every produced and a match for the H.O. Cummins output—I remember this from advertising materials when we bought the '05 Ram 3500. Of course, the later A580 5-speed and 5/6 speed transmissions became the Chrysler buzz when they were released.
That said, there is a flourishing aftermarket industry around parts for the 47RE and 48RE units. From torque converters to billet input shafts, valve body upgrades, solenoid kits, Sonnax survival items and so forth, one would quickly get the impression that this transmission was marginal from the start. That's not altogether true, though, as many of these upgrades are for radically modified, Cummins "pullers" and heavy-duty trailer haulers. Every other truck manufacturer has a laundry list of automatic transmission weaknesses to its credit—a trip to the Sonnax website uncovers an industry-wide, OEM epidemic of automatic transmission weaknesses and issues.
So let's start with a basic look at the 48RE four-speed overdrive, which shares architecture with the A727 three-speed that dates back to 1962. After doing similar homework to yours, Megatron, I concluded that if I took horse trailer pulling seriously, or pulled a 36-foot travel trailer (conventional, non-5th wheel type, as I have no bed space in the 6.3' bed with a cross bed auxiliary fuel tank), I'd have the 48RE out of the truck and on my work bench—in a heartbeat!
I would install a billet input shaft, Banks or BD torque converter, the heavy clutch and drum kits, improved bearings and thrusts, better band materials and friction clutch plates, a strut improvement plus any planetary improvements available. Then I'd add a cooler system like you describe!
I'm not building a competition puller with 800 horsepower, though, and there is a "middle road" for 48RE survival. The aim is the kind of reliability that would see our truck up and down the Alaska Highway with a 27-foot travel trailer in tow. Moderate survival measures can, as we share, be as simple as in-chassis valve body improvements, a better strut and accumulator piston (see my Sonnax upgrade article), and if removing the transmission is acceptable, at least the addition of a torque converter that will stay together.
At an even more basic level, I do have an interesting quirk that developed with the Sonnax upgrades to the valve body and accumulator piston. To keep from boring readers to tears here, I'll simply refer folks to the Sonnax upgrades article at the magazine for details: http://www.4wdmechanix.com/Survival-Upgrades-for-Jeep-and-Dodge-Ram-Automatic-Transmissions.html.
The quirk is the downshift to first gear after the transmission warms up. As you describe, there is an epidemic of solenoid issues around the RE transmissions, and the governor can also come into play on downshifting problems. The issue in this case is a hard downshift to first gear as the vehicle comes to a stop. This never occurred before the upgrades, and we purchased the truck new.
No, it's not ABS related. I clocked and calibrated the speedometer to compensate for the oversized tires, using a factory DRB III scan tool. For those interested in speedometer calibration, I cover this procedure in an HD video at the magazine site. See the 4WD Tech How-to Channel coverage: http://www.4wdmechanix.com/How-to-Dodge-Ram-Speedometer-Calibration.html.
If I come to a stop slowly, the transmission will slide somewhat "normally" into first gear. An abrupt stop can sometimes mask the hard downshift, too. But if I stop the truck at the usual pace, the downshift with the transmission warmed up will be harsh. Worth noting, there is no such symptom whatsoever when the transmission is cold.
At first, the internet scuttle had me thinking in terms of solenoids, but my instincts and experience suggested otherwise. I trained as an "Automatic Transmission Specialist" in 1969 and have rebuilt and restored everything from the Dynaflow (Buick), Hydra-Matic (big G.M. and GMC truck) and Packard Ultramatic transmissions to the modern overdrive and "electronic" truck units.
Our 48RE transmission shifts flawlessly when first started and driven for several miles—smooth up and down through the ranges and gears. It's only when the unit warms up that there is a downshift issue to first gear. All other shifts, up and down, are fine at all times, including the rollover to overdrive under heavy load. As a footnote, I adjusted the bands by the book and even rechecked these adjustments.
I tossed this topic to Mitch from Hughes Performance transmissions when we met at the SEMA MPMC Media Trade Conference in January. Mitch thought the problem could be the transmission cooler, clogged in his view. At the time, I thought about this on the 500 mile drive home...He could be right about clogging, especially when I consider the healthier line pressure from the Sonnax upgrades. There's also the thermostatic switch on the OEM cooler.
The OEM cooler in front of the radiator has a thermostat that bypasses fluid when the transmission is cold. As the transmission warms, the thermostat redirects pressurized fluid through the cooler. If the cooler were clogged, as Mitch suggested, the thermostat would force fluid into a restricted cooler. Could the harsh downshift be related to this? If so, it would be due to the back pressure boost at the governor, solenoids and valve body passageways.
Not a bad theory, and I'm somewhat buying the idea. After all, the transmission shifts up and down flawlessly when the thermostat has the fluid bypassing the cooler. I'd like to think that the trans cooler (OEM) would not clog in 90K miles, the time when I did the Sonnax upgrades. Who knows, though. If the cooler is at marginal capacity to begin with, it's possible that clogging or even the stock volume flow would create a problem with the improved fluid flow...
That's why I tossed out my question about eliminating the thermostat in the process of upgrading the transmission cooler. If anyone has an opinion about this, I'd like to hear it. I will look at the Fleece Performance cooler, Megatron. In fact, I'll likely call them, as I'd like a firsthand sense for why their approach is an improvement.
I'm totally on board with your heat/death cycle projections for automatic transmissions, old enough to remember the original Hayden add-on cooler data from the 'sixties. I was solely responsible for a 22-vehicle fleet of light- and medium-duty trucks when I did my automatic transmission certification—add-on coolers were popular then and have been ever since!
There are also the Amsoil and Mobil 1 ATF pitches from the day, emphasizing a 50-degree F drop in automatic transmission temperature through the use of synthetic ATF. That claim is real and accounts for Chrysler's progression from ATF-2 to ATF-3 to ATF-4, now a synthetic and specially formulated fluid intended to keep a modern automatic transmission alive. This is in step with your comments, Megatron. OEMs have turned to synthetic fluid and additives to get their transmissions through warranty period.
So, before I start condemning solenoids or the governor, I'll make certain that the transmission cooler is not causing a pressure spike on coast down that creates a harsh downshift to first gear. If I have an issue after that, be assured, I won't rest until the problem is resolved—by yours truly, as I'm the only one who works on my truck, especially the automatic transmission...Expect an update!